The ability to introduce and express human genes in mammalian cells has raised the possibility of a cure for inherited diseases by the replacement of defective genes. While most studies have focused on the bone marrow as a target organ for gene therapy, the lung, specifically the respiratory epithelium, is also an important target. The respiratory epithelium is involved in several common inherited and acquired diseases, including cystic fibrosis and lung cancer, and is frequently injured by environmental and occupational toxins. Furthermore, the respiratory epithelium has a large surface area and rich blood supply that may offer a means for altering the metabolism of a variety of substances and for the delivery of recombinant proteins into the circulation. We plan to develop methods for lung-directed gene therapy using respiratory epithelial progenitor cells as targets for gene transfer. The model system that will be developed consists of retrovirus-mediated transfer of the reporter gene, Escherichia coli beta-galactosidase, into isolated respiratory epithelial progenitor cells. Genetically modified progenitor cells, which will pass the marker gene to all daughter cells as a normal Mendelian gene, will then be used to repopulate denuded rat tracheas transplanted into immunodeficient mice. Features of this model that make it particularly useful are: 1) the lineage and kinetics of the progenitor cells can be determined morphologically and by histochemical staining for expression of beta-galactosidase; 2) the effect of a transferred gene on the intact epithelium, such as genes that affect respiratory epithelial regeneration or those that protect against oxidant injuries, can be studied; 3) the effect of a transferred gene on abnormal respiratory epithelial cells, such as nasal epithelial cells from cystic fibrosis patients, can be assessed since these cells will continue to express the abnormal phenotype when used to repopulate the transplanted tracheas. Once this model system is developed, lineage relationships and cell kinetics of the respiratory epithelial progenitor cells defined, and gene transfer maximized, vectors that express specific genes will be constructed. An in vivo rat model of gene transfer to the lung also will then be developed. Gene transfer to the respiratory epithelium could have been beneficial in the treatment of a wide range of respiratory and nonrespiratory diseases.